1. Field of the Invention
The invention relates to a process for preparing pyridinecarboxylic esters by reacting halogenated pyridines with carbon monoxide and a C.sub.1 -C.sub.4 -alkanol in the presence of a weak base and a catalyst. The invention further relates to a novel halopyridine as starting material for the preparation according to the invention of a novel pyridinecarboxylic ester. The pyridinecarboxylic esters preparable by the invention process have the general formula: ##STR2## wherein R.sup.1 is hydrogen, a C.sub.1 -C.sub.6 -alkyl group, a C.sub.1 -C.sub.4 -alkoxycarbonyl group or a C.sub.1 -C.sub.4 -alkoxymethyl group, R.sup.2 is a C.sub.1 -C.sub.4 -alkyl group and X is chlorine or bromine.
2. Background Art
Pyridinecarboxylic esters are important intermediates, for example, for preparing herbicides (European Published Patent Application No. 0488474) and for preparing drugs against fibrotic diseases (European Published Patent Application No. 0673932).
Processes for preparing pyridinecarboxylic esters by carbonylation reactions starting from mono- and dihalopyridines are known from the literature [European Published Patent Application No. 0282266; International Published Patent Application No. WO 93/18005; U.S. Pat. No. 4,128,554; and Shokubai, (Catalysis Society of Japan), 36, (1994), 380]. A disadvantage of these processes is that the pyridinecarboxylic esters are obtained in only moderate yields. A further disadvantage of the processes described in European Published Patent Application No. 0282266 and International Published Patent Application No. WO 93/18005 is that the dihalopyridines employed as starting material are carbonylated with low selectivity. In a further carbonylation process [J. Mol. Cat., 66, (1991), 277], high yields of pyridinecarboxylic esters are obtained starting from monohalopyridines, but the reaction requires long reaction times.